The blood-brain barrier (BBB) is composed of specialized brain microvascular endothelial cells (BMECs) that help regulate the flow of substances into and out of the brain. Intercellular tight junctions limit the passive diffusion of molecules into the brain and result in blood vessels exhibiting extremely high trans-endothelial electrical resistance (TEER) in vivo1. In addition, efflux transporters including p-glycoprotein contribute to the barrier properties by returning small lipophilic molecules capable of diffusing into BMECs back to the bloodstream. As a result, BMECs are endowed with a requisite network of specific transport systems to shuttle essential nutrients and metabolites across the BBB. In addition, because of its substantial barrier properties, the BBB has significantly hampered neuropharmaceutical development by preventing uptake of the majority of small molecule pharmaceuticals and essentially all biologics2. Conversely, breakdown and dysfunction of the BBB is associated with a variety of neurological diseases, including Alzheimer's disease, stroke, multiple sclerosis, and brain tumors3. These issues have collectively led researchers to develop a variety of BBB models to enable detailed mechanistic studies in vitro.
Most in vitro BBB models have been established using brain microvessels isolated from primary animal sources such as cow, pig, rat and mouse4. However, as a result of inevitable species differences5, 6, a robust in vitro BBB model of human origin would be of high utility for conducting high-throughput screening for brain-penetrating molecules or for study of BBB developmental, regulatory, and disease pathways in humans.
Previously, human BBB models have been established by culturing primary human BMECs isolated from autopsy tissue or, more often, freshly resected brain specimens derived from brain tumor or epilepsy patients. As a result, issues involving BMEC availability and fidelity limit the universal use of these human BBB models7. As another route toward a human BBB model, human BMECs have been immortalized8. However, immortalized BMECs suffer from poor barrier properties, including low baseline TEER9, 10 and discontinuous tight junction protein expression8, which are key hallmarks of the in vivo BBB. Collectively, these previous models have facilitated initial studies of human brain endothelium but fall short of the necessary criteria to establish a robust human in vitro BBB model.
Needed in the art is an improved BBB model.